1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for preparing the same.
2. Description of the Prior Art
Heretofore, a solid electrolytic capacitor comprises an anode, a dielectric, a semiconductor layer (solid electrolyte layer), and a cathode.
Generally, a solid electrolytic capacitor has a structure comprising an anode made of a metal exhibiting valve action (valve metal), an oxidized layer as a dielectric layer formed over the surface of the anode, a semiconductor layer (solid electrolyte layer) formed on the dielectric layer, and a cathode formed on the semiconductor layer.
In this connection, the valve metal means a metal capable of forming an oxidized layer whose thickness can be controlled by anodic oxidation. Specifically, valve metal includes niobium (Nb), aluminum (Al), tantalum (Ta), titanium (Ti), hafnium (Hf) and zirconium (Zr). Actually, however, aluminum and tantalum are mainly used.
In the following, a structure and a preparation method of a conventional tantalum (Ta) solid electrolytic capacitor will be described with reference to the drawings.
FIG. 4 is a sectional view showing a structure of a conventional tantalum (Ta) solid electrolytic capacitor.
As shown in FIG. 4, the solid electrolytic capacitor 1 using tantalum (Ta) comprises an anode body 11 which is provided with an element lead wire 11a partially inserted therein and which is formed by sintering a tantalum (Ta)-based mixed powder, a dielectric layer 12 formed over the surface of the anode body 11, an electrically conductive polymer layer 131 as a semiconductor layer 13 which is formed on the surface of the dielectric layer 12, a graphite paste layer 141 as a cathode body which is formed on the semiconductor layer 13, and a silver (Ag) paste layer 15 formed on the cathode body 14.
To the element lead wire 11a of the anode body 11 and the silver (Ag) paste layer 15, lead frames 52 are connected, respectively. The resultant is sheathed with a resin by molding with end portions of the lead frames out.
In the next place, a method for preparing a conventional tantalum (Ta) solid electrolytic capacitor will be described with reference to FIG. 5.
FIG. 5 is a flow chart showing a method for preparing a conventional solid electrolytic capacitor.
i) Preparation of Tantalum (Ta)-based Powder
To improve press-moldability, a binder is added to a tantalum (Ta) powder, and the addition is followed by mixing.
ii) Press Molding and Sintering
An element lead wire of an anode is partially inserted in the tantalum (Ta)-based powder, and the resultant was press-molded into a cylindrical or parallelepipedonal shape.
Then, the press-molded product is sintered by heating at a temperature of 1,300xc2x0 C. to 2,000xc2x0 C. under high vacuum (10.sup.xe2x88x924Pa or higher vacuum) to form a tantalum (Ta) porous body, i.e., an anode body.
The tantalum (Ta) porous body as an anode was soaked in an electrolytic aqueous solution such as a phosphoric acid aqueous solution together with a counter electrode, and a chemical conversion voltage (formation voltage) is applied to thereby form an oxidized tantalum (Ta) layer as a dielectric layer over the surface of the tantalum (Ta) porous body. (anodic oxidation method)
The thickness of the dielectric layer (oxidized tantalum (Ta) layer) is dependent upon the condition of the chemical conversion voltage (Vf: formation voltage), and characteristics as a capacitor are in turn dependent upon the thickness of the oxidized tantalum (Ta) layer. As the electrolytic solution, there may be used an aqueous solution of phosphoric acid of which concentration is adjusted to 0.6%, or the like.
On the oxidized layer formed over the tantalum (Ta) porous body in the preceding step, a solid electrolyte layer is formed as a semiconductor layer.
As the solid electrolyte, there may be used manganese dioxide, or an electrically conductive polymer obtained by polymerizing a monomeric material such as pyrrole, thiophene or a derivative thereof.
For example, when a pyrrole polymer is used as the solid electrolyte, a solid electrolyte layer is formed on the dielectric layer formed over the surface of the anode body by effecting chemical polymerization or electrolytic polymerization using a pyrrole monomer solution and a solution of an oxidizing agent such as iron trichloride, as disclosed in Japanese Unexamined Patent Publication No.2001-160318A by Fukunaga et al.
For forming the electrically conductive polymer, a process may be employed which comprises preliminarily applying an oxidizing agent to the surface of the dielectric layer, and then soaking the resultant in a monomer solution to effect polymerization reaction, as disclosed in Japanese Unexamined Patent Publication No.2000-216061A by the present inventor.
When manganese is used as the solid electrolyte, the anode body with the dielectric layer formed over the surface thereof is soaked in manganese nitrate and heat-treated. The soaking, the heat-treatment and the like are sequentially conducted to thereby form a solid electrolyte layer.
In the step of forming the semiconductor layer (solid electrolyte layer), the dielectric layer is likely to be damaged by the heat-treatment conducted in the step. It is particular when manganese is selected as a material of the semiconductor layer (solid electrolyte layer). To mend the damaged portions of the dielectric layer, the anode body with the sequentially formed dielectric and semiconductor (solid electrolyte) layers is soaked in the liquid for chemical conversion.
A graphite layer as a cathode layer is formed on the semiconductor layer (solid electrolyte layer), and a silver (Ag) paste layer is formed thereon. With respect to the formation of the graphite layer, a method disclosed in Japanese Unexamined Patent Publication No.1999-297574 by the present inventor may be employed.
Then, a lead frame for the anode is connected to the element lead wire of the anode body by spot welding, and a lead frame for the cathode is connected to the silver paste layer with an electrically conductive adhesive.
Finally, the resulting capacitor element is sheathed with a resin by molding with end portions of the lead frames out to complete a tantalum (Ta) solid electrolytic capacitor having a structure as shown in FIG. 4.
However, the tantalum (Ta) solid electrolytic capacitor prepared through the above-described steps has the following problems.
In the step of soaking the anode body, which has been soaked in the liquid for chemical conversion and is thereby provided with the dielectric layer formed over the surface thereof, in the oxidizing agent-containing solution, and air-drying the resultant, the oxidizing agent-containing solution tends to gather around the edges of the anode body with the dielectric layer formed over the surface thereof because of its surface tension. As a result, the semiconductor layer, i.e., solid electrolyte layer which is formed on the dielectric layer is likely to have a non-uniform thickness.
If the semiconductor layer (solid electrolyte layer) has non-uniformity in thickness, the semiconductor layer is liable to be damaged by heat-treatment conducted in the step of sheathing with a resin to cause separation between the layers and/or cracking of the layer.
Further, there is an undesired possibility that if the semiconductor (solid electrolyte layer) is damaged, the dielectric layer is damaged due to the damage of the semiconductor layer. This causes a drawback that leakage current (hereinafter referred to as LC) is increased by influence of heat in the step of sheathing by molding in the preparation of the capacitor, at a stage of soldering for putting the capacitor into actual use, and at a stage of actual use of the capacitor. It may be said that a capacitor of higher quality has a lower LC.
Moreover, as important characteristics of a solid electrolytic capacitor, there may be mentioned an equivalent series resistance (hereinafter referred to as ESR). ESR should be controlled at a low level as in the case of LC.
It may be mentioned as a cause of increase of ESR that the dielectric layer is not sufficiently covered with the electrically conductive polymer, and that the electrically conductive polymer layer is faultily formed. In such cases, undesirability in capacitor characteristics such as lowering in capacity and increase of dielectric loss are caused. It is, therefore, an important challenge to sufficiently cover the dielectric layer with the electrically conductive polymer.
It is the primary object of the present invention to provide a novel solid electrolytic capacitor which shows low LCs and also low ESR, and a method for preparing the same.
It is another object of the present invention to provide a solid electrolytic capacitor whose semiconductor layer as a solid electrolyte layer has a uniform thickness, a method for preparing the same.
It is a still another object of the present invention to provide a solid electrolytic capacitor whose semiconductor layer as a solid electrolyte layer has high mechanical strength, and a method for preparing the same.
It is a further object of the present invention to provide a solid electrolytic capacitor whose semiconductor layer (solid electrolyte layer) is less susceptible to damage by heat treatments in the course of preparation procedure and which is less likely to undergo occurrence of separation of layers from each other or cracking, and a method for preparing the same.
It is a still further object of the present invention to provide a solid electrolytic capacitor whose semiconductor layer (solid electrolyte layer) formed on a dielectric layer sufficiently cover the dielectric layer, and a method for preparing the same.
According to an embodiment of the present invention, the embodiment comprises a semiconductor layer including a porous phase so formed as to cover a dielectric layer and extend into voids formed during formation of an anode body and an electrically conductive polymer so formed as to fill a plurality of through-holes of the porous phase.
By virtue of the construction, the electrically conductive polymer is formed in such a manner that the pores of the porous phase are filled therewith, and the semiconductor layer, i.e., solid electrolyte layer thereby has a uniform thickness. Further, anchor effect is obtained by the penetration of the electrically conductive polymer throughout the porous phase, and the electrically conductive polymer which exhibits function as the semiconductor layer becomes less susceptible to separation from the dielectric layer and/or the cathode body. In consequence, strong bonds between the semiconductor layer and the dielectric layer and between the semiconductor layer and the cathode body are realized which are highly resistant to the thermal stress during formation of a resin sheath. This leads to increased mechanical strength of the solid electrolyte layer.
According to still another embodiment of the present invention, the embodiment comprises: a step of applying a liquid containing a substance for forming a porous phase of a semiconductor layer onto the surface of the dielectric layer, followed by drying the resultant to form the porous phase having through-holes; and a step of forming an electrically conductive polymer of the semiconductor layer in such a manner that the through-holes of the porous phase is filled with the electrically conductive polymer
By employing such a method, the porous phase serves as a skeleton for the electrically conductive polymer, and the electrically conductive polymer is stably formed substantially independently of the condition of the polymerization, and the formed semiconductor layer has a uniform thickness and high mechanical strength. In consequence, no substantial weak portions of the semiconductor layer which have low mechanical strengths are formed.